A refridgerator should be replaced every 10 to 15 years. This way they are up to date and energy efficient.
Not often if there tarting to wear and tear you should replace them.
You should replace living room furniture when it is out of style or damaged, there is no set rule.
It should not need to be replaced under normal conditions.
You should ask your dentist. You shouldn't need to replace your retainer at all unless it hurts, doesn't fit for some reason, or your orthodontist says to.
If your laptop battery fails or no longer holds a sufficient charge, you can easily replace it. But as long as it is working fine then you don't need to replace it.
you should only need to replace your fuel filter about every 45-60k miles. Check your service manual for scheduled maintenance.
You should replace your toner once a year at least since it is not really that used in a computer. Keeping your computer up to date both software and hardware will keep it running faster.
You should replace the toner cartridge in an HP7115A printer every 3-6 months depending on the cartridge quality and printing frequency, it might need to replaced sooner.
There is no definite time that you need to replace a convertible top. If it is properly maintained then it can last for many years.
when its empty
If you have a automatic transmission you need never change it as it is factory sealed for life. One less job to do. Peter
They need replaced when they wear out.